The present invention relates to cover plates for semiconductor devices and more particularly to air tight cover plates especially adapted for use as peripheral casings of packages for semiconductor devices with a high data-packaging density such as LSIs.
2. Prior Art
Until recently, the packages for semiconductor devices had been made of ceramics and plastics, but recently in their place metal cover plates have been also used. FIG. 8 illustrates a conventional cover plate of this type. A semiconductor chip 2 is mounted in a center recess of a package base 1 and is electrically connected through bonding wires 3 to wiring leads. The upper portion of the semiconductor chip 2 is hermetically sealed by a cover plate 4 which is in the form of a flat plate as shown in FIG. 10 or is a flat plate which is locally formed with a furrow-shaped reinforcing rib 4a as shown in FIG. 11. In another example of the conventional cover plates shown in FIG. 9, the cover plate 4 is not in the form of one flat plate, but its whole peripheral portion is bent at a right angle so as to define a cap-shaped cover plate 4.
Such air-tight packages are subjected to an He leakage test before they are actually used and are pressurized in an atmosphere of at least twice atmospheric pressure. When the metal covers are pressurized, the metal cover plates 4 are forced to be deformed arcuately in cross section under a pressure P as shown in FIGS. 8 and 9. The relation between the pressure applied to the cover plate and the deflection thereof will be described with reference to FIG. 13. In experiments, the cover plates were made of an Fe-N alloy 0.2 mm in thickness, and the maximum deflections at the center portion of each cover plate in atmospheres of twice and four-times atmospheric pressure were investigated. As is clear from the curves shown in FIG. 13, the larger the size of the cover plate, the greater the deflection.
When the deflection becomes too high, the cover plate 4 is forced to contact the semiconductor chip 2 or bonding wires 3 within the cover plate 4, causing malfunctioning of the chip 2.